This invention relates to hydraulic fuses or more generally to fluid fuses, for use with gases as well as liquids. Hydraulic fuses of the prior art as represented by Jackson 3,741,241 employ a valve that is biased open by a spring and is driven closed by an actuator responsive to the pressure drop across the valve. Up to a flow rate that produces a closing force that is greater than the spring force the valve acts like a fixed orifice. For a fixed orifice the pressure drop can be expressed by the following well known formula. EQU P=K(Q/CA).sup.2
where
P=pressure drop PA1 Q=flow rate PA1 C=flow coefficient PA1 A=orifice area PA1 K=dimensional constant
The flow coefficient C is highly dependent on the following factors: the shape of the orifice, the passage contours on both sides of the orifice and the fluid viscosity. When the fluid is an oil the magnitude of the variation of C due to these factors can exceed a factor of ten, which would as shown by equation (1) result in greater than a hundred fold variation in pressure drop at fixed flow rate and orifice area. Furthermore, the requirement for shut-off capability interferes with geometric simplicity in the orifice design and leads to complex flow paths and orifice shapes. The determination of area and flow coefficient of this type of orifice requires flow calibration and is therefore expensive to manufacture,. These effects cause a variability in fuse shut-off flow rate which can result in shut-offs substantially above or below the intended value. Particularly discouraging to the use of fuses in hydraulic systems is shut-off below the intended flow rate during cold start-up because of the high viscosity of cold oil.
In practical use the flow rate through a fuse is frequently limited by an upstream throttle or flow regulator to a value that is below the shut-off setting of the fuse. Under this limiting rupture of the hose does not produce a sufficient flow increase to result in fuse shut-off. In order for a fuse shut-off under this circumstance it must have adequate response to the short decompression flow pulse that follows rupture. High responsiveness of a fuse to a short pulse of high flow rate is dependent on three characteristics of its moving element: low friction, high force response to flow rate and high ratio of bias spring rate to mass of the moving parts. The large diameter sliding pistons characteristics of the prior art are not conductive a very low friction or very low mass. In particular, the valve configurations of the prior art require the bias springs of the lowest possible spring rates in order to obtain large valve area changes with minimum flow rate change. The use of an accumulator between the throttle and the fuse to increase the decompression pulse duration relieves the fuse response requirement but can introduce an unacceptable lag into the operating system.